<!DOCTYPE html>
<html lang="en">

<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
</head>

<body>
  <script>
    //声明提升
    //  console.log(age);
    //  var age=10
    //  fun()
    //  function fun(){
    //   console.log(9);
    //  } 
    var age
    function fun() {
      console.log(9); 9
    }
    console.log(age); //undefined
    age = 10
    fun()
  </script>
</body>

</html>